Communications systems often use directional antennas to focus transmitted and received power in a desired direction. The direction of maximum transmitted and received power is sometimes referred to as the “boresight” of the antenna. Highly directional or narrow beamwidth antennas must be pointed accurately to ensure the antenna boresight is oriented in the desired direction. For example, for communication between two moving platforms, narrow beamwidth antennas are constantly adjusted as the pointing angles change due to relative movement of the platforms. Additional stabilization may also be required to accommodate changes in platform orientation, for example, using an inertial reference that relates the platform orientation (roll, pitch, and yaw/heading) to the earth.
Antenna pointing can be performed “closed loop,” where the antenna pointing angles are continuously adjusted to maximize received (or transmitted) signal power. Closed loop pointing systems typically require complex feed arrangements or complex receiver signal processing, which can increase cost. Hence, in many applications, closed loop pointing is undesirable.
Antenna pointing can also be performed “open loop,” where the antenna pointing direction is determined algorithmically and the antenna is directed to point in a particular direction. Typically, open loop antenna pointing algorithms translate platform and target locations from inertial coordinates, determined by an inertial reference unit or attitude reference system, into relative ship's coordinates of azimuth and elevation which is provided to the antenna. In open loop pointing, the antenna is therefore calibrated to ensure that the boresight is accurately directed towards the specified azimuth and elevation.
Various sources of error can occur in an antenna system which results in boresight errors. For example, antennas are often stabilized with respect to an attitude reference system to compensate for attitude changes (roll, pitch and yaw) of the platform. For example, stabilization for platform orientation sometimes involves providing an attitude reference system at or near the antenna. The attitude reference system can be, for example, a set of accelerometers or a complete inertial navigation system. There can, however, be alignment errors between the axes of the antenna system and the axes of the attitude reference system. These alignment errors result in misalignment or boresight error between the desired and actual antenna pointing direction. Replacement of components in the antenna or attitude reference system can make recalibration of the boresight necessary. Other potential sources of boresight error include mechanical tolerances in the antenna positioning equipment, mechanical tolerances in the antenna beam forming elements, and electrical tolerances.
Traditionally, boresighting an antenna has been performed by holding the platform to which the antenna is mounted stationary (fixed to the earth) while the antenna is pointed at a fixed calibration target. This approach is undesirable in many circumstances. For example, a ship cannot be held in a stationary position while at sea due to sea swell, and thus is returned to port to boresight an antenna. Similarly, an aircraft is usually required to land in order to perform boresighting. This can prove inconvenient, for example in military applications, since either the mission must be interrupted or the antenna operated without the boresight calibrated. Furthermore, requiring the calibration target to remain in a fixed position can prove inconvenient. For example, this has typically meant that calibration targets are land based, again making it difficult to boresight an antenna on a ship at-sea.